High temperature, solid oxide electrolyte fuel cell configurations, and fuel cell generators, are well known in the art, and are taught by Isenberg, in U.S. Pat. Nos. 4,395,468 and 4,490,444. These fuel cell configurations comprise a plurality of individual, series and parallel electronically connected, axially elongated, generally tubular, annular cells. Each cell is electronically connected in series to an adjacent cell in a column, through narrow cell interconnections extending the full axial length of each cell. This interconnection contacts the air electrode of one cell and the fuel electrode of an adjacent cell, through a metallic coating and a fiber metal felt.
Each fuel cell is formed on a long, electronically insulating, porous support tube, generally made of calcia stabilized zirconia, which provides structural integrity for the fuel cell. Surrounding, and deposited on this support tube, is a thin, porous air electrode, generally about 500 microns to 2000 microns thick, deposited by well known techniques. The air electrode can be comprised of doped or undoped oxides or mixtures of oxides in the pervoskite family, such as LaMnO.sub.3, CaMnO.sub.3, LaNiO.sub.3, LaCoO.sub.3, LaCrO.sub.3, and the like. Generally surrounding the outer periphery of the air electrode is a gas tight, solid electrolyte, usually of yttria stabilized zirconia. Substantially surrounding the solid electrolyte is an outer, porous, fuel electrode, usually of nickel-zirconia cermet. A single, open end, oxidant injector tube is used in each fuel cell, to flow gaseous oxidant into the cell. The oxidant contacts the support and diffuses through it to the air electrode.
Many improvements have been made to the support tube for the fuel cell. Ruka et al., in U.S. Pat. No. 4,596,750, provided a fluorite type support tube material which would be better able to resist cracking due to migration of lanthanum or manganese materials contained in the contacting air electrode, at fuel cell operating temperatures of about 1000.degree. C. Rossing et al., in U.S. Pat. No. 4,598,028, provided lighter weight, thinner, higher strength support tubes, which utilized a ceramic powder and ceramic fiber combination, allowing reduction of the oxygen path length to the air electrode through the support. Improvements have also been made to the air electrode, for example, Ruka, in U.S. Pat. No. 4,562,124, taught introduction of small amounts of cerium into the air electrode material, to provide a better match of coefficient of thermal expansion with the support tube.
A problem with the air electrode however, is that electronic current flow through the air electrode to the interconnection, which connects the next cell electronically in series, was confined to the circumferential path of the air electrode around the non-electronically conductive cell support tube, and the enclosed central oxidant chamber. This current path resulted in a circumferential voltage gradient, and did not provide for complete uniformity in cell current density. In addition, it has always been difficult to match the thermal expansion coefficients of the support and contacting air electrode, and to prevent some migration of air electrode material into the support at the 1000.degree. C. operating temperature of the cell in a fuel cell generator. What is needed is a new design for the fuel cell, eliminating the problems of circumferential current flow, thermal mismatch, and material migration, while still providing a strong support for the air electrode, electrolyte and fuel electrode.